Short Answer:

Residual stresses can be relieved by applying various mechanical and thermal methods that allow the material to return to a stable internal stress condition. The most common and effective method is heat treatment, where the material is heated to a specific temperature and then cooled slowly to release locked-in stresses.

In simple words, residual stresses can be removed by stress-relieving heat treatment, vibration stress relief, shot peening, or mechanical stretching. These methods help redistribute or reduce internal stresses without affecting the mechanical properties of the material. Relieving residual stresses improves dimensional stability, fatigue strength, and service life of the component.

Detailed Explanation:

Relief of Residual Stresses

Residual stresses are internal stresses that remain in a material after manufacturing processes such as welding, casting, machining, rolling, or heat treatment. These stresses can cause warping, cracking, or dimensional instability during use. Hence, it is essential to relieve or reduce them before a component is put into service.

The process of removing or reducing residual stresses is known as stress relief. Stress relief ensures that the material regains structural stability and performs reliably under operational conditions. Different methods are used depending on the type of material, magnitude of residual stress, and component design.

The following are the most common and effective methods used to relieve residual stresses.

1. Heat Treatment (Thermal Stress Relief)

Heat treatment is the most widely used method for relieving residual stresses. It involves heating the material to a specific temperature below its transformation range and then cooling it slowly.

Process:

1. The component is gradually heated to a predetermined temperature (usually 500°C to 650°C for steel).
2. The temperature is maintained for a certain time to allow uniform temperature distribution throughout the material.
3. The component is then cooled slowly in a controlled environment (such as a furnace) to avoid new stress formation.

Mechanism:
At high temperatures, atoms in the crystal lattice gain mobility and can rearrange themselves, allowing internal stresses to relax.

Advantages:

• Removes most residual stresses completely.
• Restores dimensional stability.
• Improves fatigue and corrosion resistance.

Example:

• Welded pressure vessels and large machine frames are often stress-relieved by furnace heating.

Note: The temperature and duration of heat treatment depend on the material type:

• Carbon steel: 500°C – 650°C
• Cast iron: 450°C – 550°C
• Aluminum alloys: 150°C – 200°C

2. Annealing

Annealing is a heat treatment process used not only for softening materials but also for relieving internal stresses. It involves heating the material above its recrystallization temperature and then cooling it slowly.

Process:

1. The component is heated to a specific temperature (depending on material composition).
2. It is held at that temperature for a certain period to allow atomic rearrangement.
3. It is then cooled slowly in the furnace.

Effect:

• Eliminates residual stresses completely.
• Improves ductility and reduces hardness.
• Refines the grain structure of the material.

Example:

• Used for cold-worked materials such as rolled sheets, drawn wires, and machined parts to remove strain hardening effects.

3. Vibration Stress Relief (VSR)

Vibration stress relief is a mechanical method used to reduce residual stresses without using heat.

Process:

1. The component is mounted on a vibration machine.
2. Controlled vibrations are applied at specific frequencies for a certain time.
3. The vibrations cause micro-yielding at highly stressed zones, allowing stresses to redistribute.

Advantages:

• Suitable for large structures that cannot be placed in a furnace (e.g., heavy machine beds, large weldments).
• Saves time and energy compared to heat treatment.
• No risk of thermal distortion or oxidation.

Limitations:

• Does not completely remove residual stresses; only redistributes them.
• Less effective for materials with very high internal stresses.

Example:

• Used for large welded frames, pressure vessels, and ship hulls.

4. Mechanical Stretching or Overloading

This method involves subjecting the material to controlled mechanical loading to yield the highly stressed regions, thereby redistributing the residual stresses.

Process:

1. The component is loaded slightly beyond its elastic limit.
2. Plastic deformation occurs in highly stressed regions, equalizing the stress distribution.

Applications:

• Commonly used in aerospace components and large forgings.
• Aluminum sheets used in aircraft are often stretched to remove forming stresses.

Advantages:

• Effective for large structures where heating is impractical.
• Does not alter the material’s properties significantly.

Limitations:

• Requires precise control of applied load to avoid permanent damage.

5. Shot Peening (Surface Stress Relief)

Shot peening is a cold working process used mainly to relieve tensile residual stresses at the surface by inducing beneficial compressive stresses.

Process:

• The surface of the component is bombarded with small spherical steel or ceramic balls at high velocity.
• Each impact plastically deforms the surface layer, creating compressive stresses that counteract tensile residual stresses.

Benefits:

• Improves fatigue life and resistance to stress corrosion cracking.
• Commonly used for springs, gears, turbine blades, and aircraft components.

Limitations:

• Effective only for surface stresses, not internal ones.

6. Aging (Natural or Artificial)

Aging is another method used to relieve residual stresses, especially in aluminum and magnesium alloys.

Process:

• The material is allowed to rest at room temperature (natural aging) or heated slightly (artificial aging).
• This allows atomic diffusion and rearrangement, which gradually relieves internal stresses.

Applications:

• Common in aircraft-grade aluminum alloys after forming or machining operations.

7. Localized Heating

In some cases, localized heating is applied to relieve stresses in specific areas such as weld joints.

Example:

• In welding, post-weld heat treatment (PWHT) is used to relieve stresses in and around the weld zone.
• A torch or induction heater may be used to apply heat locally.

Advantage:

• Cost-effective and fast for small areas.
• Prevents distortion of the entire structure.

8. Design and Process Modifications

Sometimes, residual stresses can be minimized during the design or manufacturing stage itself by:

• Using uniform cooling rates in casting and welding.
• Avoiding sharp corners and sudden cross-section changes.
• Using preheating before welding to reduce temperature gradients.
• Selecting materials with low thermal expansion coefficients.

Conclusion

Residual stresses can be relieved through thermal, mechanical, or vibrational methods depending on the type of material and application. The most common technique is heat treatment, where the material is heated and slowly cooled to allow stress relaxation. Other methods like vibration stress relief, shot peening, and mechanical stretching are also effective under specific conditions. Relieving residual stresses enhances dimensional accuracy, prevents cracking, and improves the fatigue life of mechanical components, ensuring their safe and long-term performance in engineering applications.